Pleistocene sea-floor fibrous crusts and spherulites in the Danakil Depression (Afar, Ethiopia)

Pleistocene fibrous aragonite fabrics, including crusts and spherules, occur in the Danakil Depression (Afar, Ethiopia) following the deposition of two distinctive Middle and Late Pleistocene coralgal reef units and pre-dating the precipitation of evaporites. Crusts on top of the oldest reef unit (Marine Isotope Stage 7) cover and fill cavities within a red algal framework. The younger aragonite crusts directly cover coralgal bioherms (Marine Isotope Stage 5) and associated deposits. Their stratigraphic position between marine and evaporitic deposits, and their association to euryhaline molluscs, suggest that the crusts and spherules formed in restricted semi-enclosed conditions. The availability of hard substrate controls crust formation with crusts more often found on steep palaeo-slopes, from sea level up to at least 80m depth, while spherules mainly occur associated with mobile substrate. Crusts reach up to 30cm in thickness and can be microdigitate, columnar (branching and non-branching) or non-columnar, with laminated and non-laminated fabrics. Two different lamination types are found within the crystalline fabrics: (i) isopachous lamination; and (ii) irregular lamination. These two types of lamination can be distinguished by the organization of the aragonite fibres, as well as the lateral continuity of the laminae. Scanning electron microscopy with energy dispersive X-ray spectroscopy analyses on well-preserved samples revealed the presence of Mg-silicate laminae intercalated with fibrous aragonite, as well as Mg-silicate aggregates closely associated with the fibrous aragonite crusts and spherules. The variety of observed fabrics results from a continuum of abiotic and microbial processes and, thus, reflects the tight interaction between microbially mediated and abiotic mineralization mechanisms. These are the youngest known isopachously laminated, digitate and columnar branching fibrous crusts associated with a transition from marine to evaporitic conditions. Understanding the context of formation of these deposits in Afar can help to better interpret the depositional environment of the widespread Precambrian sea-floor precipitates.